Surgical tourniquets are widely used during surgical procedures to occlude the flow of blood in a portion of a limb during the procedure, particularly in connection with arthroscopic procedures relating to the hand, wrist, elbow, foot, and knee, in which the existence of a bloodless field in the appropriate portion of a patient's limb may be required. Surgical tourniquets are similarly useful in other procedures in which the creation of a bloodless field is desirable, including nerve grafting and harvesting. It is important that pressure be maintained by a surgical tourniquet despite the manipulation by a surgeon of the limb on the elevation of blood pressure due to an autonomic nervous response in which blood flow is being occluded, where the manipulation tends to affect the pressure within the tourniquet and the pressure distribution of the tourniquet on the extremity. Also, it is important that the tourniquet pressure be minimized to reduce the potential of adverse effects due to the constriction of the tourniquet. It is also important that the surgical tourniquet be easy to use and physically stable so that the surgeon may focus his attention on other aspects of the surgery.
Typical non-invasive blood pressure measurements are generally made by either of two methods. In the first method, a stethoscope is placed beneath or below a pressure cuff. The pressure cuff is then inflated to a pressure in excess of the patient's systolic blood pressure, causing occlusion of the blood flow past the pressure cuff. The pressure in the cuff is then gradually decreased, while a person taking the pressure measurement listens through the stethoscope. As the blood pressure overcomes the pressure in the cuff, blood begins flowing past the cuff. This blood flow causes sounds called Korotkoff sounds to be generated. These sounds are distinctive of the blood flow past the pressure cuff, and abate once the blood pressure fully reopens the arteries constricted by the pressure cuff. By identifying the Korotkoff sounds, and monitoring the decreasing pressure in the cuff, the pressure at which blood pressure exceeds cuff pressure can be identified. Determinations can be made of both systolic and diastolic blood pressures by monitoring Korotkoff sounds associated with a higher pressure (systolic) and a lower pressure (diastolic). This method is sometimes referred to as the auscultatory method.
A second method of non-invasive blood pressure measurement, called oscillometric measurement, is also accomplished using a pressure cuff and decreasing pressure. In oscillometric pressure measurement, however, a column of mercury is generally used to measure the pressure in the cuff. As the patient's blood pressure exceeds the cuff pressure, blood flow past the cuff causes the pressure in the cuff to pulse in time with the blood flow past the cuff. The mercury column being used to measure pressure thus also pulses, allowing a person taking the blood pressure measurement to visually identify blood flow past the pressure cuff. The patient's blood pressure can thus be determined by monitoring the mercury column for the start of pulsing, and the pressure at which the pulsing begins.
Recently, measurement of oxygen levels in blood (such as used in pulse oximetry) has been shown to be a reliable indicator of blood flow past a tourniquet. As the effectiveness of the tourniquet decreases, the oxygen level of blood downstream of the tourniquet increases. This effect may occur before the flow is sufficient to create flow noises, such as those utilized for auscultic or oscillometric determination of blood flow past a tourniquet. This effect is sometimes referred to as “pinking up” of the blood.
Blood oxygen levels may be determined non-invasively through photometric methods which use reflected and/or refracted light to evaluate oxygenation of the blood. Monitors for determining oxygenation may be placed against the skin of a patient to whom a tourniquet has been applied, allowing measurement of the oxygen saturation level of the blood at that site to be measured.
The pressure necessary to occlude blood flow in a tourniquet is dependant on the blood pressure of the patient on whom the tourniquet is applied. When the pressure of the tourniquet exceeds the blood pressure, the heart of the patient is unable to pump blood past the tourniquet location. Typically, the tourniquet pressure cuff is inflated in excess of the blood pressure to ensure occlusion of blood flow past the tourniquet. When a surgical tourniquet is applied to the arm, over-pressurization of the surgical tourniquet of 50-75 mm Hg is common. When a surgical tourniquet is applied to a leg, over-pressurization of the surgical tourniquet of 75-100 mm Hg is common.
Over-pressurization of a tourniquet can cause injury to a patient on whom the tourniquet is being used. Neural and vascular injuries can occur due to the pressure in use causing physical trauma. Accordingly, the pressure used to occlude blood flow past the tourniquet should be kept to the minimum necessary to overcome the systolic pressure, thus preventing the pumping of blood past the tourniquet. Contrarily, high pressures ensure occlusion of blood flow.
In addition to the constraints of simply using a surgical tourniquet to occlude blood flow, the use of the surgical tourniquet to provide a barrier to allow intravenous regional anesthesia (IVRA) raises additional concerns, particularly with the dangers associated with the flow of the anesthesia past the barrier and into the body of the patient. Furthermore, the need to flush metabolized anesthesia from the limb at the conclusion of an operation requires the allowance of systolic flow to both test veins and arteries potentially affected by the surgery, and to displace remaining anesthesia and its metabolized by-products.